1. Field of the Invention
The invention relates to signal correlation, and in particular to reducing complexity of matched filter for correlating multi-path interference in a receiver.
2. Description of the Related Art
A wireless communication signal suffers adverse multipath effect during transmission when encountering obstructions, buildings, and geographical features such as mountains, to result in two or more multipath components of the transmitted signal reaching a receiver at different times, causing constructive and destructive interference, and phase shifting.
A spread spectrum system is designed to combat multipath interference by multiplying narrowband data by a wideband pseudo noise (PN) code in a spread spectrum transmitter to produce a wideband transmitted signal with spectrum of nearly the spectrum of the PN code. A spread spectrum receiver such as a RAKE receiver resolves multipath interference by correlating the incoming data with several timely shifted PN codes to detect multipath components, and adding the multipath components to yield an output data.
In spread spectrum systems such as Wideband Code Division Multiple Access (WCDMA) systems conforming to the UMTS/IMT-2000 (Universal Mobile Telecommunications System/International Mobile Telecommunications-2000) specifications, downlink (base stations to subscriber terminals) signals within a cell are modulated by a cell-specific scrambling code, also referred to as PN code, prior to transmission. Typically, orthogonal channelization codes, also known as spreading codes, are assigned to distinct physical channels transmitted in a cell, thus creating orthogonal downlink signals within the cell.
FIG. 1 is a block diagram of a conventional multipath searcher 1, comprising input data memory 100 and a plurality of correlators 110, 111, 112, 113, 114, 115, 116, 117, 118, and 119.
Data memory 100 receives and stores data A1, A2, A3, . . . An+10, represent a composite of multipath components, which are received CDMA data. Correlator 110 has correlation length n, time offset τ0, and receives pseudo noise (PN) code B1 to Bn to correlate data A1 to An to produce correlation output 120. Correlators 111 to 119 receive PN code B1 to Bn shifted by corresponding time offsets τ1 to τ9 to produce correlation outputs 121 to 129. Multipath searcher 1 compares correlation outputs 120 to 129 to identify the largest as the most significant multipath component of received signal. Multipath searcher 1 has a search range 130 (τ9-τ0), indicating a range of the time offsets to be searched.
Multipath searcher 1 shown in FIG. 1 employs many correlators to detect multipath components in input signal over a wide multipath search range, resulting in high circuit complexity and manufacturing cost. Thus a need exists for an apparatus and method to provide a multipath component that yields simple circuit implementation with low manufacturing cost.